The present invention relates to certain non-toxic alkoxy substituted aryl onium salts, such as diaryliodonium salts, and their use in making UV curable non-toxic coating compositions. More particularly, the present invention relates to a method of treating various metal and plastic substrates with non-toxic UV curable oxirane containing materials to render them suitable for food packaging.
Prior to the present invention, metallic and plastic substrates used in food packaging were generally coated with non-toxic curable compositions, such as heat curable epoxy-phenolics. Organic solvent solutions of various organic resins also have provided an alternative non-toxic surface in food applications. However, presently available procedures are economically unattractive because of the high input energy required, or environmentally unsatisfactory due to the presence of organic solvents. The food industry is constantly seeking alternative methods for packaging food. The preferred procedures would be less energy intensive and faster under continuous conditions.
UV curable resins, such as solventless epoxy resins, have many desirable characteristics. For example, such resins are fast curing, environmentally safe, and require little energy to effect cure. As taught by British patent 1,516,351 or 1,516,352, or U.S. Pat. No. 4,058,401, Crivello, incorporated herein by reference, diaryliodonium salts and triarylsulfonium salts are very effective photoinitiators for epoxy resins. However, the use of triarylsulfonium and diaryliodinium salts as photoinitiators has been limited, because those materials, for example, diphenyliodonium hexafluororoantimonate are extremely or very toxic materials.
As used hereinafter, the term "toxicity" or "very toxic", or "extremely toxic" will be used in accordance with the definitions shown in Clinical Toxicology of Commercial Products, fifth edition, (1984) of Robert E. Gosselin et al., Williams & Wilkins, Baltimore, on page 2, Section 1. An oral dose of 5-50 mg/kg (weight of animal tested) would be lethal to 50% of the test subjects and would be classified as extremely toxic, while a 50-500 mg/kg dosage would be classified as very toxic. Although Gosselin et al. refers to human usage, comparable studies with laboratory animals have shown that diphenyliodonium hexafluoroantimonate exhibits an Acute Oral Toxicity (LD.sub.50) (TSCA 7/79) of 25 mg/kg. The comparable 4-methoxy substituted analog exhibits a 296-385 mg/kg rating over a three day period.
For handling purposes in industry, a rating of 5000 mg/kg or 5 g/kg or greater would be considered "nontoxic". For example, table salt has a rating of 3750 mg/kg.
In addition to toxicity, the aforementioned hexafluorometalloid diaryliodonium salts also have been found to be substantially incompatible with solventless epoxy resins particularly oxirane containing organosiloxanes.
The present invention is based on our discovery that certain alkoxy substituted aryl onium salts have been found to be non-toxic, particularly diaryliodonium hexafluorometalloid salts, having at least one nuclear bound-OR group attached by a carbon-oxygen linkage to an aryl nucleus of the diaryliodonium hexafluorometalloid salt, where R is an alkyl radical having a value of at least 8. It has been further found that when the diaryliodonium hexafluorometalloid salt is substituted with an --OR group, where R has a value of 10-20, the resulting salt is a readily, synthesizable crystalline material having a melting point in the range of 50.degree. to 100.degree. C., while salts having --OR groups where R is C.sub.8 or less are more difficult to purify, since they are oils.